Conversion of cholesterol to bile acids in the liver is the major pathway for disposal of cholesterol in mammals. Recent studies have shown that bile acids not only are the physiological detergents that facilitate the absorption, transport and distribution of lipid-soluble vitamins and dietary fats, but also are the signaling molecules that activate nuclear receptors; farnesoid X receptor (FXR), pregnane X receptor (PXR) and vitamin D3 receptor (VDR). Bile acid feedback inhibits bile acid synthesis by suppressing the gene encoding the rate-limiting enzyme of the classic bile acid biosynthetic pathway, cholesterol 7alpha-hydroxylase (CYP7A1). It has been suggested that FXR induces a negative nuclear receptor, small heterodimer partner (SHP), which subsequently inhibits CYP7A1 gene transcription. Bile acids also induce inflammatory cytokines, which activate the MAP kinase/JNK signaling pathways to inhibit CYP7A1 gene transcription. Sterol 27-hydroxylase (CYP27A) catalyzes sterol side-chain oxidation of the classic pathway and initiates the alternative bile acid synthesis pathway. CYP27A1 also catalyzes VD3 synthesis in the liver, intestine and kidney. Oxysterol 7alpha-hydroxylase (CYP7B1) plays roles in steroid and oxysterol metabolisms, in addition to bile acid synthesis. This project will study regulation of the human CYP7A1, CYP27A1, and CYP7B1 genes by bile acids and steroids. Molecular biology techniques including small interference RNA, reporter gene assay, site-directed mutagenesis, electrophoretic mobility shift assays, mammalian two-hybrid assay, and real time PCR will be used to study gene regulation. Specific aim 1 will study the mechanism of bile acid inhibition by PXR, VDR, and MAP kinase/JNK pathways. Specific aim 2 will study the mechanism of cytokine and VDR regulation of the CYP27A1 gene. Specific aim 3 will study regulation of the CYP7B1 gene by steroid response element binding proteins (SREBP) and estrogen receptor (ER). The long-term objective of this research project is to understand the molecular mechanisms of regulation of bile acid synthesis and steroid metabolism, and to elucidate the mechanism of human diseases such as cholestatic liver diseases, gallstone disease, atherosclerosis, and diabetes. Drug therapies targeted to the nuclear receptors involved in regulation of bile acid synthesis genes could be developed based on this research for lowering serum cholesterol and treating cardiovascular and liver diseases.